Many thermal devices generate a signal in response to a given environmental condition. For instance, a thermocouple has a voltage output due to a temperature difference from one end to the other end of the thermal device. Thermocouples are analog temperature sensors that utilize the thermoelectric properties of two dissimilar materials, typically metals, to generate an electromagnetic force (EMF) in proportion to a temperature gradient across the non-homogenous conjunction of the two materials. Common thermocouples used in temperature measurement comprise two metal wires of different thermoelectric properties called thermoelements connected at one end to form a “hot junction,” also known as a “measuring junction.” The other ends of the wires are connected to instrumentation such as a voltmeter to measure the EMF produced by the thermocouple. The wires are connected to the instrumentation at a known reference temperature to form a “reference junction” or a “cold junction”.
There are many thermal errors associated with applying thermocouples and other such temperature sensors. For example, there are errors unique to the specific sensor, and application errors relating to the way the sensor is positioned or mounted in the user's application in the “field” (mounting error). Positioning or mounting error is often a major source of temperature sensor error in the user application. For instance, if the sensor mounting depth is shallow or if a temperature gradient exists near the tip of the sensor, there will be a heat flow “Q”, which significantly affects the sensor output. This error can easily be whole degrees or tens of degrees and be the largest error in a thermal system.
In general, these error classifications in temperature sensors relate to the thermal heat transfer characteristics within the sensor and to the environment surrounding the sensor. For instance, a sheathed sensor measuring a thermal process has a heat flow from the process to the sensing element comprising a boundary layer heat flow from the process to the sensor sheath, a heat flow from the sheath to the sensing element, and other heat flow paths from the sheath to the cold end of the sensor. Each of the heat flow paths indicated provide another source of potential thermal error in the thermal sensing system if not adequately accounted for or otherwise compensated by the measurement system.
A sensor that is adequately immersed thermally into a process environment would have negligible difference in temperature between the sensing element and the process conditions. However, a sensor that is not adequately in contact with the process could have significant thermal errors. For instance, a surface of a hot plate at 500° C. may be surrounded by room air so that just slightly above the surface of the plate would be air near room temperature. A surface sensor in such a condition would have a very sharp thermal gradient from the surface of the target to slightly above the sensing element (near ambient temperature) and so it is very likely that a difference exists between the sensing element and the target. In addition a probe placed onto a surface would act like a heat sink and locally cool the target further adding to the measurement error.
Although some current sensing approaches use calibration data to improve performance and some systems can be compensated by thermal analysis on a particular system, the thermal environment, mounting, application, and sensor specific errors still exist degrading thermal sensing system accuracy, process variability, and product quality.
Accordingly, there is a need for improved compensation for such thermal transfer paths in a thermocouple system and other temperature management systems. The improved compensation should minimize static and dynamic system errors by mitigating variations in the sensor output due to thermal transfer between the sensor and the thermal environment surrounding the sensor, and due to the thermal mounting error of the sensor in the user application. The improved compensation should also mitigate variations due to sensor specific errors particular to the sensor's construction.